1. Field This invention relates to a method for treating coal, and more particularly to a method for separating resin from coal by froth flotation.
2. State of the Art Using froth flotation to remove and separate natural resins ("resins") from resin-bearing coal is well known. U.S. Pat. No. 1,773,997 (Green) is illustrative of the process. Resin-bearing coal is comminuted in the presence of two parts of water and passed through a 40 mesh screen. Another part of water is added, and the resulting pulp is introduced into a froth flotation machine. A frothing agent, such as amyl alcohol, is added to the pulp, and the mixture agitated. A froth forms which carries with it a certain portion of any present resin while the coal-predominant tailing remains.
The resultant resin-bearing froth is then retreated. The resinous froth is reintroduced into the flotation machine, water is added to fill the machine to a working level, and substances such as potassium alum, may be added. The resulting mixture is then agitated. More frothing agent is introduced, and a resin froth is taken from the flotation machine. Green reported that relatively pure (approaching 96% by weight) resin can be attained using the above two-step method.
Extracting resin from resin-bearing coal is particularly important in Western North America. There, certain coal fields contain significant quantities of resins. Other areas of the world known to have resin-bearing coal are Mainland China and Argentina.
The State of Utah has significant amounts of resin-bearing coal in its coal fields. The resins obtained from Utah coal generally have low specific gravities, approximately 1.03 g/cm.sup.3, and vary in color from lemon yellow to almost black. The resins are important commercially, being used for adhesives, varnishes, coatings, waterproofing, linoleum, etc., and generally command a substantially higher price per pound than coal. This resinous coal also has a significantly lower sulfur and ash content than coal obtained elsewhere, for example, coal from the eastern United States.
Prior art flotation techniques for resin recovery have not, however, been particularly selective. Both components of the feed, resin and coal, have similar hydrophobic characteristics as shown by the contact angle and bubble attachment time data listed in Table 1. The difference between the bubble attachment times for resin and coal is insufficient to achieve the desired selective flotation separation by conventional flotation techniques.
The hydrophobic character of resin and coal from the Hiawatha, Utah seam is shown in Table 1. The bubble attachment time is measured as the bed induction time with particle size 212.times.300 microns at a pH of 6.5.
TABLE 1 ______________________________________ Contact Angle Bubble Attachment Component Degrees Time (ms) ______________________________________ Resin 58-59 5 Coal 48-51 15 ______________________________________
Some work has been done by others, U.S. Pat. Nos. 4,537,599 (Greenwald) and 4,543,104 (Brown), in treating coal with ozone during froth flotation to remove unwanted coal contaminants such as ash and sulfur. Brown teaches the treatment of coal with ozone for a period of five to twenty minutes, and preferably at least ten minutes, to obtain a very hydrophobic coal which floats, leaving ash behind as part of the gangue.
Greenwald also teaches the use of ozone in froth flotation to separate coal from the contaminants pyrite and clay. Greenwald also seeks very hydrophobic coal and his process leaves the contaminants behind as gangue. Since the hydrophobic characteristics of coal and resin are so similar, treating resin-bearing coal in the manner of Greenwald or Brown yields no separation of the resin particles from the coal particles. The present invention overcomes the insufficiencies of the prior art froth flotation techniques to yield effective separation of coal particles from resin particles.